1. Field of the Invention
The present invention relates to a planar light source device and a liquid crystal display apparatus using the same.
2. Description of the Related Art
Typical liquid crystal display apparatus used for personal computers and mobile phones include a liquid crystal panel and a planar light source device. The liquid crystal panel has two substrates with a liquid crystal layer interposed therebetween. The planar light source device is mounted on the back of the liquid crystal panel. Conventionally, a linear cold cathode fluorescent lamp has been widely used as a light source of the planar light source device. Recently, however, a light emitting diode (LED) has become increasingly used because of longer life, better color fidelity, and mercury-free operation. When using a monochromatic LED, it is necessary to use different colors of LEDs to produce white light. Often used are three colors of LEDs: red (R), green (G), and blue (B). A liquid crystal display apparatus uses a plurality of each color LEDs to achieve necessary luminance. The number of each color LEDs may be different depending on necessary luminance and desired white point for each of the R, G, and B. The liquid crystal display apparatus further has a light guide plate to guide and diffuse the light emitted by the LEDs evenly over the plate. The plurality of LEDs are placed in a row at the side or bottom of the light guide plate. The light guide plate mixes the three colors of light together to create white light and have the light incident on an exit surface thereof.
The planar light source device using the LEDs, however, has the following problems. Since each color LED has different temperature characteristics, the luminance of each LED varies by changes in ambient temperature. Further, the luminance varies also by changes in time. The luminance varies not only among R, G, and B LEDs but also between the same color LEDs. The luminance variation among the LEDs is significant at an early stage.
The luminance variation among the LEDs generates variation in the luminance or chromaticity of the entire planar light source device. In order to suppress the luminance or chromaticity variation, a planar light source device having an optical sensor at an edge of a light guide plate has been developed. Such a planar light source device is described in Japanese Unexamined Patent Application Publication No. 11-260572, for example. The planar light source device having an optical sensor detects the light from LEDs with the optical sensor and controls the LEDs to keep the luminance constant by feedback based on the detected light. The optical sensor has three types of optical sensor elements for detecting the wavelength corresponding to red, green, and blue light, respectively. Sending feedback to each color LED allows producing desired white light. The luminance and chromaticity of the light from the light sources are thus kept stable to improve display quality.
The planar light source device performing feedback control using the optical sensor, however, has a problem. The problem will be explained hereinafter with reference to FIGS. 8A and 8B. FIG. 8A is a top plan view of a conventional planar light source device. The planar light source device in FIG. 8A has LEDs 1, a light guide plate 2, a reflector 3, and an optical sensor 4. The LEDs are light sources emitting red, green, and blue color light. The light emitted by the LEDs 1 enters the light guide plate 2 directly or after reflected by the reflector 3. The light travels through the light guide plate 2, is diffused evenly over the plate, and then exits from the light guide plate 2 through an exit surface which faces toward a liquid crystal panel (not shown). The optical sensor 4 for detecting the light from LEDs 1 is mounted at the side of the light guide plate 2 to make the display device thinner. The optical sensor 4 has three types of optical sensor elements for detecting red, green, and blue light, respectively. The feedback control is performed to maintain a constant luminance based on detection signals from the optical sensor 4.
The light emitted by the LEDs 1 is directional. The emission angle distribution, for example, is such that the light intensity is highest at the normal to the LED and decreases as an angle off the normal increases. In the light guide plate 2, the light incident on the side surface of the plate at a smaller angle than a given angle is totally reflected. The light incident at a greater angle than a given angle, on the other hand, exits from the light guide plate 2. Therefore, the intensity of the light exiting through the side surface of the light guide plate 2 reflects the emission angle distribution of the light from the LEDs. Thus, the obliquely incident light from a specific LED 1 shown by an arrow in FIG. 8A has high intensity. The optical sensor 4 therefore detects the light greatly affected by a specific LED 1. As a result, the luminance variation of a specific LED 1 is detected as the luminance variation of the entire light source device. Feedback control based on such a detection result from the optical sensor 4 fails to keep the luminance and chromaticity of the entire planar light source device constant. This problem is more significant when the optical sensor 4 is placed on the side surface of the light guide plate opposite to the side surface where the LEDs 1 are mounted since the light intensity is highest at the normal to the LED 1. Accordingly, the planar light source using the LEDs 1 and having the optical sensor on a surface which is neither a display surface nor the surface opposite to the display surface is incapable of accurate feedback. The luminance and the chromaticity therefore vary with temperature or time, resulting in varying emission characteristics. A liquid crystal display apparatus employing such a planar light source device has varying display characteristics and deteriorated display quality.
As described above, the conventional planar light source device and liquid crystal display apparatus are incapable of accurate control of light from a light source.